Plymouth is a city in Marshall County, Indiana, United States. The population is 10,214 in the 2020 census. It is the county seat of Marshall County. Plymouth was the site of the first retail outlet of defunct U.S. retailer Montgomery Ward in 1926.

The Yellow River is a 62.3-mile-long (100.3 km) tributary of the Kankakee River in the Central Corn Belt Plains ecoregion, located in northern Indiana in the United States. Via the Kankakee and Illinois rivers, it is part of the watershed of the Mississippi River, draining an area of 427 square miles (1,110 km2).